1. Field of the Invention
The present disclosure relates generally to medical ventilator systems, and more particularly, to a medical ventilator with autonomous control of oxygenation.
2. Description of the Related Art
Achieving adequate oxygenation is one of the primary goals of mechanical ventilation. This goal is accomplished through the adjustment of the fraction of inspired oxygen concentration (FlO2), positive end-expiratory pressure (PEEP), and mean airway pressure. Titration of these variables is guided by continuous noninvasive monitoring of oxygen saturation by pulse oximetry (SpO2) and intermittent arterial blood sampling for arterial oxygen tension (PaO2) and measured oxygen saturation (SaO2). Traditional application of this technology has sought to maintain the SpO2 between 98 and 100% to assure adequate oxygenation and prevent desaturation events (SpO2<88%) which has been shown to have a deleterious effect on patient outcome. Previous work has focused on preventing hypoxemia and hyperoxia which has tended to maintain the SpO2 on the flat portion of the oxyhemoglobin dissociation curve. Maintaining the SpO2 in this area allows for transient changes in PaO2 caused by shunting while maintaining SpO2 and therefore the O2 concentration in the blood.
Goals for oxygenation vary with specific disease states and philosophy of the attending heath care personnel. In adults, adequate oxygenation is typically considered an SaO2>90% and PaO2>60 mm Hg. In neonates, where both hypoxia and hyperoxia are associated with adverse outcomes, arterial oxygen is more tightly controlled. Similarly, in patients with head injuries, even a single hypoxemic event is associated with poor outcome. In these patients, hyperoxia may be indicated both to prevent hypoxemia and to assure adequate brain tissue oxygenation. Previous automated control methods have required the clinician to enter an SpO2 target and range of acceptable values. These methods presuppose the attending clinician is capable of determining what is best for the patient and adjusting the device accordingly.
Oxygen toxicity is a common concern in intensive care, but the effects of elevated FlO2 in critically ill patients is controversial. A common goal of PEEP titration is reduction of FlO2 to nontoxic levels (e.g., <0.60); however, the exact level of O2 that may be construed as safe is unknown. In adults, the potential effects of hyperoxia are always preferable to the immediate effects of hypoxemia.
Although maintenance of adequate O2 delivery and prevention of hypoxemia are the primary goals for the battlefield casualty, military medical operations have unique concerns. In civilian US hospitals, under normal conditions, O2 reserves are plentiful. In military medical and flight operations, O2 is a limited resource that needs to be conserved. During deployed military medical operations, the logistical considerations required to provide and sustain O2 resources in forward areas are considerable. It has been estimated that O2 containers and O2 generation equipment comprise approximately fifteen to thirty percent of the entire logistical footprint of weight and space (cube) necessary to provide medical care in the field during combat operations. These considerations become even more acutely focused during transport when both the patient, medical equipment and O2 need to be moved in air and ground vehicles that are called into service to move the casualty. Mission length can vary depending upon the tactical and strategic consideration(s) of the theater and the criticality of the patient. The necessity of accounting for potential required O2 resources as well as the physical transport of O2 tanks and liquid O2 is an important component of any mission. Mission planning commonly involves a calculation of required O2 needs and then doubling that value as a margin of safety. Similar O2 supply problems exist with mass casualty incidents and disaster relief. These applications have needs that are very similar to the military's. Immediately following hurricane Katrina for example, affected area hospitals ran out of O2 as did the few ambulances that supported the evacuations from the area. Florida had similar situations following its hurricanes: (1) the loss of electrical power prevented hospitals from distributing gaseous O2 from their liquid O2 tanks, (2) downed electrical lines prevented restoration of power which prevented the use of O2 concentrators and, (3) power lines and trees blocked roads which prevented deliveries of bottled gaseous O2. Also, rural areas that routinely transport patients over great distances to hospitals sometimes take in excess of 4 hours which could marginalize supplies of on-board O2.
Given the preceding considerations, it would seem apparent that O2 conservation takes on greater importance in prehospital and transport environments. In these environments the goals of O2 therapy are to prevent hypoxemia, to assure adequate arterial oxygenation and to minimize O2 usage.